Immunoglobulin G (IgG) from patients with heparin-induced thrombocytopenia binds to platelets in an environment that contains approximately equimolar concentrates of platelet factor 4 (PF4) and heparin. The binding requires the F(ab1)2 end of the IgG but not the Fc end. The specific IgG binding to platelets can be isolated by binding to PF4 that is attached to heparin-sapharose.